ARM & AArch64: use table for EmitCommonNeonBuiltinExpr

This extends the intrinsic lookup table format slightly, and adds
entries for use the shared ARM/AArch64 definitions. The benefit is
currently smaller than for the SISD intrinsics (there's more custom
code implementing this set), but a few lines are saved and there's
scope for future expansion.

llvm-svn: 201848